Retroviruses frequently undergo genetic alteration by point mutation or by recombination to generate variants which may exhibit different infectious and pathogenic properties. The generation of these variants and their role in pathogenicity is a major focus of this project. Mice infected with ecotropic murine leukemia viruses (MuLV) recombine with members of a very closely related group of endogenous retroviral env-gene sequences to generate polytropic viruses. Polytropic viruses utilize a different cellular receptor and exhibit a host range distinct from the parental ecotropic MuLV and have been implicated in the induction of a number of proliferative diseases. Inbred mouse strains contain in their genomes 30 to 40 endogenous sequences that are closely homologous to the env genes of polytropic MuLVs. We have determined that at least several of these sequences participate in the generation of recombinant retroviruses, however, their participation is clearly non-random. We have recently described two major antigenic subclasses of polytropic MuLVs which correspond to recombination with distinct endogenous sequences. Different inoculated ecotropic MuLVs consistently yield distinctly different ratios of the antigenic subclasses. One of our current objectives is to elucidate the basis of this selectivity. To this end we have defined a small region encoding the nucleocapsid gene and a portion of the protease gene which strongly influences the types of polytropic MuLVs that are generated in infected mice. A second objective of this project is to elucidate the effects of viral interactions on the dynamics of virus spread and pathology in mixed retrovirus infections such as that generated by the emergence of genetic variants. Phenomena such as pseudotyping and viral interference are consequences of mixed viral interactions, but the potential for these interactions to confer synergism on the components of the infection is not well understood. Our initial studies of mixed virus infections considered ecotropic and polytropic viral interactions during leukemogenesis by Moloney (M) MuLV. This work suggested a stepwise mechanism of oncogenesis facilitated by pseudotyping and interference. Recently our efforts have been directed at the effects of mixed retrovirus infections generated by co-inoculation of ecotropic and polytropic MuLV isolates. We have found a striking synergism between the two co-inoculated viruses, which results in a rapidly fatal neurological disease occurring within 10 to 15 days after infection. The level of the polytropic MuLV infection is highly elevated in co-inoculated mice compared to mice infected with the polytropic MuLV alone. This enhancement in replication is likely the result of nearly complete pseudotyping of the polytropic MuLV genome within ecotropic virions throughout the course of infection. Polytropic genomes are extensively pseudotyped at the earliest times after infection when virus spread in the host is minimal. Considering that pseudotyping can only occur in cells infected by both viruses, this observation indicates that both of the inoculated viruses initially infect the same small population of cells. Examination of the level of infection and the degree of pseudotyping at the earliest times that virus can be detected in co-inoculated mice, revealed a converging infection of the ecotropic and polytropic MuLVs in a small population (<1%) of periferal blood cells. FACS analyses suggest that this population is of the monocyte/macrophage lineage.